# 2: Atoms, Molecules, and Chemical Reactions


The atomic theory, first proposed in modern form by John Dalton, is one of the most important and useful ideas in chemistry. It interprets observations of the every-day world in terms of particles called atoms and molecules. Macroscopic events—those which humans can observe or experience with their unaided senses—are interpreted by means of microscopic objects—those so small that a special instrument or apparatus must be used to detect them. (Perhaps the term submicroscopic really ought to be used, because most atoms and molecules are much too small to be seen even under a microscope.) In any event, chemists continually try to explain the macroscopic world in microscopic terms.

To get a sense for just how small the atoms we will be working with in the next chapter are, check out this Ted-Ed video called 'Just How Small is an Atom'.

• 2.1: Prelude to Atoms and Reactions
The atomic theory, first proposed in modern form by John Dalton, is one of the most important and useful ideas in chemistry. It interprets observations of the every-day world in terms of particles called atoms and molecules. Macroscopic events—those which humans can observe or experience with their unaided senses—are interpreted by means of microscopic objects—those so small that a special instrument or apparatus must be used to detect them.
• 2.2: Macroscopic Properties and Microscopic Models
As a simple example of how the macroscopic properties of a substance can be explained on a microscopic level, consider the liquid mercury. Macroscopically, mercury at ordinary temperatures is a silvery liquid which can be poured much like water—rather unusual for a metal. Mercury is also the heaviest known liquid. Its density is 13.6 -fold greater than water. When cooled below –38.9°C mercury solidifies and behaves very much like more familiar solid metals such as copper and iron.
• 2.3: The Atomic Theory
• 2.4: Macroscopic and Microscopic Views of a Chemical Reaction
• 2.5: Testing the Atomic Theory
To test a theory, we first use it to make a prediction about the macroscopic world. If the prediction agrees with existing data, the theory passes the test. If it does not, the theory must be discarded or modified. If data are not available, then more research must be done. Eventually the results of new experiments can be compared with the predictions of the theory.
• 2.6: Atomic Weights
The relative masses of the atoms are usually referred to as atomic weights. The atomic-weight scale was originally based on a relative mass of 1 for hydrogen. As more accurate methods for determining atomic weight were devised, it proved convenient to shift to oxygen and then carbon, but the scale was adjusted so that hydrogen’s relative mass remained close to 1. Thus nitrogen’s atomic weight of 14.0067 tells us that a nitrogen atom has about 14 times the mass of a hydrogen atom.
• 2.7: The Amount of Substance- Moles
"How much?" in the above sense of the quantity of atoms or molecules present is not the same thing as "how much" in terms of volume or mass. The International System of Measurements (IUPAC) has a measure of amount that reflects the number of atoms present, and it is called the mole.
• 2.8: The Mole
The very large numbers involved in counting microscopic particles are inconvenient to think about or to write down. Therefore chemists have chosen to count atoms and molecules using a unit called the mole. One mole (abbreviated mol) is $$6.022 \times 10^{23}$$ of the microscopic particles which make up the substance in question.
• 2.9: The Amount of Substance
In the International System this quantity is called the amount of substance and is given the symbol n.
To obtain such a pure number, we need a conversion factor which involves the number of particles per unit amount of substance. The appropriate factor is given the symbol $$N_A$$ and is called the Avogadro constant.
• 2.11: The Molar Mass
It is often convenient to express physical quantities per unit amount of substance (per mole), because in this way equal numbers of atoms or molecules are being compared. Such molar quantities often tell us something about the atoms or molecules themselves.
• 2.12: Formulas and Composition
When a reaction is carried out for the first time, little is known about the microscopic nature of the products. It is therefore necessary to determine experimentally the composition and formula of a newly synthesized substance. One way to approach this involves quantitative analysis—the determination of the percentage by mass of each element in the compound. Such data are usually reported as the percent composition.
• 2.13: Balancing Chemical Equations

Thumbnail: Spinning Buckminsterfullerene ($$\ce{C60}$$). (CC BY-SA 3.0; unported; Sponk).

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